1. Field of the Invention
This invention relates to a catheter or probe with a sensor for placement within a human or animal to allow direct monitoring within the body, and more particularly, relates to rezeroing of the pressure sensor in vivo and a method of rezeroing the sensor in vivo.
2. Background
Catheters have been inserted into humans and animals for diagnosis, monitoring and treatment purposes and such catheters have to be small and flexible in size and structure in order to function without irritating the body part into which they are placed. Typically, catheters are used to infuse medications or remove samples for purposes of analysis. Multilumen catheters are sometimes used to infuse medication and remove samples at the same time.
If a sample is removed for purposes of analysis, it has to be taken to a laboratory, analysis made and the results transmitted to the doctor. Delay in performing the analysis and transmitting the data sometimes can be fatal to the patient. Another use of a catheter is to form a hydraulic column for transmitting pressure readings to an external sensor. In connection with pressure sensors, the hydraulic column has problems of air bubbles, kink in the tubing of the column and blood clots, each of which tend to affect the reliability, waveform fidelity, the accuracy and precision of the readings.
Current technology uses a 20 gauge catheter to introduce therapy or provide diagnosis. This size is easily inserted and easy to use without irritation or injury to the body. Twenty gauge catheters are commonly used on all but pediatric patients without problems of introduction or irritation in connection with peripheral vessels. A pressure sensor, on the distal tip of a 20 gauge catheter or probe would eliminate the mentioned hydraulic column difficulties.
Catheter tip pressure sensors have heretofore been rezeroed by zeroing ex vivo, by placing a known pressure on each side of the sensor. Those rezeroing techniques have difficulties in connection with accuracy, infection control and ease of use. The sensor may include a semiconductor with a pressure responsive circuit located on the tip of a catheter.
Catheters having sensors are known and include sensors mounted at the distal tip of the catheter. U.S. Pat. No. 3,710,781 shows a catheter tip pressure sensor wherein a pair of elongate pressure sensor elements are mounted on opposite sides of a support. This is done to permit as large a sensor area as practical for purposes of providing accurate reproductions of blood pressure waveforms. U.S. Pat. No. 3,545,275 shows a device responsive to impedance used for measuring pressure with a miniaturized sensor. The sensor is responsive to diaphragm fluctuations where the diaphragm is mounted in the distal end of a small diameter tube. A small probe is disclosed in U.S. Pat. No. 3,811,427 wherein a pair of electrodes are mounted in a liquid filled chamber and are sensitive to fluctuations in a diaphragm mounted at the distal end of a catheter tube. The probe is said to be smaller than one millimeter. Two embodiments are shown. Each has a diaphragm in the distal end of the catheter and a longitudinal separator which carries the pressure responsive means and isolates the liquid from the remainder of the catheter such that fluctuations in the diaphragm are transmitted to the separator which is generally longitudinally disposed.
U.S. Pat. No. 4,722,348 shows a semiconductor mounted within a tubular housing in the end of the catheter tube and having a pressure inlet. Sealant protects the semiconductor which is held to the support by the double face adhesive tape which also carries the electrical conductors. U.S. Pat. No. 4,809,704 discloses catheters with the sensor mounted in the tip of the catheter supported on a base by a potting resin carried within the catheter tube. The resin is a urethane or silicone material about the sensor with appropriate openings for sampling. Assembly of the sensors within the catheters has been slow and labor intensive.
The offset pressure due to changes in atmospheric pressure has to be accounted for in that the reference side of the sensor is considered an essentially zero pressure. U.S. Pat. No. 4,672,974 has an apparatus with a port for a substitute reference pressure and an external pressure gauge for measuring the mean pressure through an auxiliary lumen of the catheter. Thus, a known pressure can be substituted for atmospheric pressure in the process of rezeroing the offset pressure. U.S. Pat. No. 4,712,566 has a sensor carried on a guide which is moveable in and out of the catheter tube so that the in vivo side of the sensor may be brought into the catheter removed from blood pressure and subjected to a generated pressure during calibration. The guide and catheter cooperate with one another to seal off the sensor during rezeroing.
U.S. Pat. No. 4,854,326 has an impedance variable transducer with a technique for zeroing the in vivo transducer by varying the static pressure in a reservoir connected to the transducer. Thus, changes in the height of the reservoir can be used to adjust the zero point of the transducer. This approach, although feasible, introduces another variable into the system. In addition, a gas retaining flexible membrane has to be located in the liquid filled lumen to the reservoir. The membrane is to separate the liquid from the gas filled lumen. The flexible membrane is said to prevent oscillating movement of the liquid in the lumen as a result of the interchange of energy by the liquid, the displacement of the diaphragm in the transducer and the compliance of the lumen about the liquid. Isolation is not the purpose of the flexible membrane.